The geology of the Porirua District

Abstract

Article 1. The Lower Mesozoic alternating greywackes and argillites in the Porirua district exhibit the following sedimentary features; graded bedding, lamination, convolute lamination, cross lamination, mud flakes and current markings. The sedimentary features, petrography and palaeontology of these rocks are described. The sediments, rapidly derived from a Lower Mesozoic plutonic terrain, have been rapidly transported and deposited by turbidity currents. The orientation of cross lamination and current markings suggests that turbidity currents transported sediment NNE along the axial belt of the geosynclinal trough, sub-parallel to the regional tectonic trend. At Porirua, sedimentation was in a deep water, poorly circulated trough situated a considerable distance from the supplying landmass. Article 2. The well indurated greywacke-argillite rocks of the Porirua district have been subjected to strong folding, followed by more than one period of intense faulting and fracturing. The main axis of flexural folding trends NNE-NE and plunges 14°-17°NE. Numerous small-scale folds are described. They exhibit a wide range of forms; from open, symmetrical and faulted folds, to isoclinal folds. A few localized folds, described as anomalous, have axes that cut across the NNE-NE trending regional fold axis. Faults are classified, on the basis of certain field criteria, into two broad age groups, early and late. The early faults include those associated with folding, and those formed during subsequent episodes of faulting. Late faults comprise the "major" transcurrent Owhariu Fault and many smaller displacements. The late faults have been active during, and possibly somewhat prior to the Quaternary. Article 3. Titahia corrugata, a new genus and species of agglutinating annelid (Terebellidae), is described from the greywacke-argillite rocks of Wellington. Article 4. The stratigraphy of some Pleistocene and Holocene superficial deposits in the Porirua Basin is described. Deposits in nine sections are correlated by means of red weathering supplemented with lithology, erosion intervals, altitude, and radiocarbon dates. Three levels of red weathering occur in a number of sections: the lowest and most intense red weathering (zone A) corresponds to a long, warm, moist period (?Last Interglacial); the middle and upper, less intense red weatherings (zones B and C) correspond to shorter, warm, moist periods. Deposits are subdivided into three "formations", FI, FII, and FIII. FI - the beds deposited before zone A red weathering, and including two solifluction layers and clays. FII - the beds deposited after zone A red weathering, but before zone C red weathering, and including clayey-silts, a solifluction layer, estuarine deposits beneath the 60 ft terrace, and carbonaceous beds more than 41,000 years old. FIII - the beds deposited after zone C red weathering, and including solifluction and soliflual valley-fill between 23,000 and 20,000 years old, alluvium beneath the 25 ft terrace, wind-blown sands, clayey-silts and loams. The sequence of Pleistocene and Holocene events is summarized.